fault codes 122 and 998, high idle etc

Thanks in advance for any assistance. I just picked up a 92 F150 Flareside 4x4 with a 351 with EFI which has 263K on the clock. I have an idle that is around 1300 rpms or so with some variance. It is also an absolute pig on gas, coming in under 10 mpg. The tranny also shifts extremely harsh in first, almost as if someone has hit me from behind. According to the guy I purchased it from the tranny had been rebuilt and the shop put in some sort of kit or adjustment that was supposed to make it shift harsher to actually extend the life of the tranny and help with towing (is this b.s., or plausible? the guy was completely upfront about everything the truck needed, so I tend to believe that he was telling the truth). The check eng light was on when I bought it and the guy said he had brought it to AutoZone and they told him it needed a new TPS. Without checking to verify this myself, I stupidly just purchased and replaced the TPS. No change in my performance or issues at all. I checked the codes this time for myself and if I am remembering correctly it did come up with 122 for a TPS problem, and also came up with 998, some sort of hard fault problem. Does this mean there is a problem with the computer, or with wiring somewhere? When I looked up the code 998 I find some info that says 'problem with ECT/ACT/MAP...etc'. If it is a problem with one of those sensors, how do I find it? Also, for a little more background, the guy had taken the thermostat completely out, but even when the truck warms up to full temp, it still runs with all those issues, no change. My airbox screws are also broken, as is probably common on many F series trucks (my last one was that way also). One more note...I read that the ACT is the air charge temp sensor and is located in the intake. Not sure if it makes any difference, but when I took the throttle plate, body, whatever you call it, off to replace the TPS, it was cover with a massive amount of what I am assuming is dirt and oil or carbon like build up, so maybe that ACT sensor is not able to get a reading because it is covered in sludge? Thanks for any help.

sean